There is considerable demand for short-wavelength laser sources such as green, blue and UV lasers. One known approach to create such a light source is to utilize red or infra-red laser diodes, which are widely available in a variety of configurations. These diodes, in combination with nonlinear elements made of optically nonlinear materials, can produce short-wavelength frequency-doubled radiation by means of second harmonic generation (SHG) in the nonlinear element.
A laser source for frequency doubling requires (a) high power, (b) stable, narrow-line operation, (c) some a means of fine-tuning the spectrum to match it to a doubling material, and, importantly (d) simple, low-cost optics and assembly.
A number of prior art designs for frequency doubling of laser diode emission have been disclosed. For example, U.S. Pat. No. 5,384,797, in the names of Welch et al., describes a monolithic multi-wavelength laser diode array having output light that can be coupled into a ferroelectric frequency doubler integrally formed on an array substrate. U.S. Pat. No. 5,644,584, in the names of Nam et al. describes a tunable blue laser diode having a distributed Bragg reflector (DBR) or distributed feedback (DFB) tunable diode laser coupled to a quasi-phase-matched waveguide of optically nonlinear material. U.S. Pat. No. 6,370,168 to Spinelli describes an intracavity frequency-converted optically-pumped semiconductor laser based on surface-emitting gain-structure surmounting a Bragg mirror, and an external concaved mirror. U.S. Pat. No. 6,393,038 to Raymond et al. describes a frequency-doubled vertical-external-cavity surface-emitting laser; and, U.S. Pat. No. 6,438,153 to Caprara et al. describes an intracavity-converted optically-pumped semiconductor laser.
Although these aforementioned inventions appear to perform their intended function, they provide solutions wherein power and frequency stabilization requirements are met through the use of complex laser structures or complex nonlinear element arrangements. Furthermore, since complex laser designs usually lead to somewhat reduced power, these prior art solutions either use an intra-cavity nonlinear doubling arrangement to benefit from the intra-cavity resonance power enhancement, at the expense of yet more complex cavity control, or use single-pass doubling with relatively low output powers
An alternative approach is to use semi-conductor, high power, lasers of simple cavity design, such as edge emitting 980 nm laser diodes commonly used to pump erbium-doped fiber amplifiers, in an external cavity arrangement with frequency stabilization provided by an external frequency selective reflector. Lasers of this type are commercially available and typically use a substantially broadband fiber Bragg grating (FBG) as the frequency selective reflector. These single spatial mode semiconductor chips have an antireflection coated front facet, and can generate over 1 watt of power in continuous operation, provided that optical feedback from the FBG into the laser diode is optimized, typically at a feedback level when about 3% of the laser radiation is returned back into the laser diode.
However, the substantially broadband reflection spectrum of the FBG results in a broad laser linewidth of the order of 0.3–1 nm. This linewidth far exceeds typical linewidth requirements of ˜0.02–0.1 nm or less for efficient SHG in such materials as periodically-poled LiNbO3, thus necessitating the use of other external frequency selective elements, such as bulk diffraction gratings.
As an additional advantage over FBGs, a bulk grating arrangement avoids the optical loss associated with coupling of a laser diode beam into an FBG, therefore significantly increasing the output optical power available for nonlinear frequency conversion.
An external diffraction grating is most commonly used in either Littrow configuration, wherein the grating is oriented to retro-reflect a portion of laser radiation back into the laser diode, or in a Littman configuration, wherein a retro-reflected beam is formed by a portion of the laser beam after reflecting twice from the grating and once from a mirror. Several implementations of these configurations have been disclosed in U.S. Pat. No. 5,392,308 to Welch et al., U.S. Pat. No. 5,771,252 to Lang et al. and in U.S. Pat. No. 5,867,512 to Sacher.
FIG. 1A illustrates a conventional Littrow configuration. In this configuration, a laser diode 10 is combined with a rotatable reflective grating 12, as indicated by arrow 14, via appropriate optics 16 to provide frequency selection feedback for laser diode 10. Tuning of the optical frequency in this configuration is achieved by rotation of the reflective grating 12.
In U.S. Pat. No. 5,448,398, Asakura, et al. disclose an intra-cavity nonlinear frequency doubling in a Littrow cavity with a reflective diffraction grating that incorporates a selective filter to provide noise reduction.
However, the low optimal feedback requirements of ˜3% required for high-power lasers make the intra-cavity placement of the nonlinear SHG element an unnecessary complication. An outside-the-cavity arrangement becomes more practicable, wherein a nonlinear element is disposed in the path of an optical beam generated by the external cavity laser.
The drawback of a conventional Littrow cavity design described above for an outer-cavity arrangement of the nonlinear element is that the rotation of the reflective grating also leads to angular sweeping of the output beam 22 as indicated by the arrow 25 at twice the rate of grating rotation. When the cavity is followed by an angular selective element such as nonlinear frequency-doubling crystal, this angular tuning of the output beam can yield a drop in SHG efficiency and an undesirable steering of the beam.
This deficiency of the conventional reflective grating based Littrow configuration had been overcome in a now, more common Littman type configuration, illustrated in FIG. 1B. In this configuration, laser diode 10 is combined to form a “folded” external optical cavity with a fixed reflective element grating 12 and a rotatable reflective element 18, as indicated by arrow 20, to provide frequency selection feedback for laser diode 10. The output zero-order beam 22 is reflected off the fixed grating and does not change direction during tuning. The first order beam 13 incident upon the mirror and is reflected back to the grating, which diffracts it further back into the laser diode thereby providing frequency-selective feedback. Therefore in the Littman-type arrangement, the retro-reflected beam experiences two first-order diffractions off the grating, as opposed to a single diffraction in the case of Littrow arrangement. In US patent No. RE35,215, Waarts et al. describe a semiconductor laser light source which employs a Littman grating coupled to a back facet, providing short wavelength light by means of frequency doubling of red or infrared light from a high power flared resonator type laser diode, or a MOPA (master oscillator power amplifier) type laser diode.
The Littman arrangement however is known to have higher optical loss resulting from the double diffraction of the retro-reflected light. This higher loss can be a significant drawback for frequency doubling, since the SHG efficiency is proportional to optical power squared, and the frequency-doubled intensity suffers disproportionably when optical power is decreased. Indeed, if a 3% feedback is required from the grating-mirror arrangement for optimal operation of the laser diode, the first order diffraction efficiency is required to be √{square root over (3%)}=17.3%, resulting in the zeroth-order output beam at only 83% of the facet power not including any other optical losses. In frequency-doubling, output efficiency is proportional to power squared and 17% loss of optical power reaching the SHG element results in a 32% reduction in frequency-doubled light intensity.
This invention provides a Littrow-type extended cavity arrangement obviating many of the aforementioned limitations of the prior art solutions, while providing for the aforementioned desirable attributes of a frequency-doubled laser; more particularly, high power, stable, narrow-line operation, a means of fine-tuning the spectrum to match it to a doubling material, and simple, low-cost optics and assembly.
An object of this invention is to provide a simple Littrow-type external cavity configuration for a diode laser, which substantially maximizes output power and enables frequency tuning without angular tuning of the output beam.
Another object of this invention is to provide an external cavity laser diode arrangement for generation of short-wavelength radiation through frequency doubling, which combines high power, frequency stabilization and tuning capabilities using simple, low cost optics and assembly.